The present invention generally relates to noise figure measurement of optical amplifiers.
Optical amplifiers are commonly used in optical communication systems. One of the parameters that is important in characterizing an optical amplifier is the amplified spontaneous emission (ASE) in the presence of an optical signal. The measurement of ASE is important for determining a noise figure of the optical amplifier as well as for an ASE buildup in communication systems, where ASE can limit performance. The ASE represents a noise signal that is generated within the optical amplifier and is amplified by the amplifier. The ASE typically has a much wider bandwidth than the optical signal.
When no signal is present, the amplifier generates and amplifies only the ASE. However, when an optical signal is present, the ASE level is reduced in comparison with the ASE level in the absence of an optical signal due to amplifier gain reduction. The gain reduction depends on the amplitude and wavelength of the optical signal. Thus, in order to accurately characterize amplifier performance, the ASE must be measured at an optical signal level and wavelength that corresponds to normal operation.
A known technique to perform an ASE measurement at an actual signal wavelength is called time-domain extinction technique (compare e.g. U.S. Pat. No. 5,340,979). A tunable laser source is gated on and off with a fast (&lt;1 .mu.s) transition time. As the source signal is switched on, the optical amplifier output momentarily peaks and then returns to its steady state power level. The gated-on-time needs to be long enough such that the optical amplifier stabilizes to its steady state. Once the optical amplifier is in its steady state, the switch rapidly extinguishes the signal incident on the optical amplifier. Immediately after the signal is gated off, the ASE level at the amplifier output will be identical to the true ASE level without the deleterious effects of the stimulated and spontaneous emission generated by the laser source. Then the ASE level rises to a level that corresponds to the unsaturated state of the optical amplifier.
The ASE transient is recorded either with an optical spectrum analyzer (OSA) or, e.g., in conjunction with an oscilloscope connected to the analog output of the OSA. For the portion of the ASE transient missed after the signal was gated off, extrapolation can be used to determine the desired ASE power density. However, the time-domain extinction technique requires two highly blocking optical switches for the incident and the outgoing laser beams of the amplifier having a short switching time and switching both beams synchronously with high accuracy.
In the time-domain extinction technique, due to the relatively long carrier lifetime in the optical amplifier, the amplifier remains in essentially the same saturation condition during the short off-period. This way, the ASE can, in principle, be measured without the disturbing influence of the amplified signals and the source spontaneous emission (SSE) that accompanies them. However, there are several limitations of the time-domain extinction technique:
a) Even though the signals may be completely switched off, the photodetector does usually not fully recover from the high power state (signal) to the low power state (ASE). A noticeable error from this effect can be the result. PA1 b) In practical implementations of the time-domain extinction technique, the sources are repetitively switched off and on with a duty cycle of 50%, thereby reducing the maximum achievable power by 50% (equivalent to 3 dB). PA1 c) Pulsing the sources may interact with the power stabilization control loop of the amplifier. PA1 a) Measure the SSE power of each laser at the wavelength of channel 3. PA1 Multiply the SSE powers with the transmissions through the respective attenuators (weighting). PA1 Add the weighted SSE powers, to obtain the total SSE input power of channel 3. PA1 b) Measure the gain of channel 3 (=signal output power/signal input power). PA1 c) Determine the total spontaneous emission, ASE+G.times.SSE of channel 3 by interpolation. PA1 d) Subtract G.times.SSE from the total spontaneous emission, ASE+G.times.SSE, to obtain the ASE.
Another possibility for ASE measurement of optical amplifiers is the ASE interpolation/subtraction technique. To explain the principle of this measurement, the typical output spectrum of an optical amplifier in a single channel condition is shown in FIG. 1. The spectrum consists of the amplified narrow-band input signal P.sub.signal, the ASE power P.sub.ASE and the amplified source's spontaneous emission power G.times.P.sub.SSE, where G is the gain of the amplifier and P.sub.SSE is the power of the source's spontaneous emission (SSE). The latter two are spectrally wide. For such spectrally wide sources, the displayed power is equal to their spectral power density multiplied with the given spectral bandwidth of the optical spectrum analyzer.
The basis of the noise figure is the precise measurement of the spectral power density of the ASE at the signal wavelength. Practically, however, the ASE cannot be measured at the signal wavelength. Instead, the total spontaneous emission (P.sub.ASE +G.times.P.sub.SSE) is interpolated from two (as in FIG. 1) or more samples to the right and the left of the signal. In the ASE interpolation/subtraction technique, G and P.sub.SSE are measured separately. Then the product of G and P.sub.SSE is subtracted from the total spontaneous emission, to obtain only P.sub.ASE.
As a third possibility, EP 0 678 988 A1 discloses a method of measuring the ASE level in the presence of a signal at a signal wavelength comprising the steps of detuning the signal to a second wavelength different from the signal wavelength, determining a difference function corresponding to the difference in ASE levels before and after detuning of the signal as a function of wavelength, measuring the ASE level at the signal wavelength and adding the value of the difference function at the signal wavelength to the ASE level measured in the previous step.
In contrast to the single-channel excitation, FIG. 2 shows an output spectrum of an optical amplifier in a wavelength-division multiplexing (WDM) situation with four channels. In this example, the optical amplifier is driven by four narrow-band laser sources of different wavelengths. To determine the noise figure of each channel, the ASE at the signal wavelengths must be determined. This is possible by using the time-domain extinction technique or the ASE interpolation/subtraction technique. In the example of FIG. 2, each of the four ASE values is determined on the basis of interpolating the two samples to the left and the right of each channel. The gains G and the spontaneous emissions SSE of all sources are determined separately, to be able to subtract G.times.SSE.
If, for example, the ASE of channel 3 is to be determined, then the following procedure is typically used. All measurements are based on optical spectrum analysis.
The situation becomes more complicated when the spacing between the individual channels becomes very narrow (dense WDM). In this case, it is often not possible to place samples between the channels, because of the limited resolution bandwidths of typical optical spectrum analyzer.